Which one is correct?
This pretest-posttest quasi-experimental study was made on 236 male dormitory students who had been recruited to the study through systematic random sampling.
This pretest-posttest quasi-experimental study was made on 236 male dormitory students who had been recruited into the study through systematic random sampling."
The verb recruit normally means to persuade someone to work for a company or become a new member of an organization, especially the army.
The word to would normally be taken as an infinitive marker for the verb saying what people have been recruited to do.
Young children have been forcibly recruited to perform combatant roles - Bound to cooperate
The word into is a preposition that identifies the group that people are joining.
.. significant numbers of children have been recruited into armed groups such as the RUF. Research methods in conflict
In the case of research studies, the subjects become members of the panel, and so recruited into is the appropriate term.
Here is a similar sentence about the conduct of research studies which uses the word admit rather than recruit: this should not affect the preposition.
The purpose of the research and, in summary form, the eligibility criteria that will be used to admit subjects into the study (e.g., adults on medication for high blood pressure, diabetic patients on insulin; normal, healthy adults; etc.) - Subject selection, recruitment and compensation
